People often say you should avoid exercising for weeks after COVID because this may trigger Long COVID (LC). (For whatever reason, there isn't much scientific data on this.)

I have a theory on what's happening and how to mitigate the risk.

Disabling cases of LC generally involve ME/CFS. The most distinctive symptom of ME/CFS is post-exertional malaise (PEM).

PEM facts:

1. When you have ME/CFS, you experience PEM if you exert yourself too much. PEM can happen with a delay up to 72h.

2. There tends to be a relationship between how hard you exert yourself and how severe/long-lasting the PEM is. And there's a relationship between how severe your ME/CFS is and how easily you trigger PEM.

3. People often completely recover from PEM within days. But sometimes it takes weeks/months. Sometimes it causes a semi-permanent decrease in functioning. Conversely, some patients find that if they avoid PEM for enough months/years, their energy slowly returns over time.

e.g. Let's say Alex has "mild" ME/CFS. He can handle 10min of cardio without triggering PEM. One day, he does 30min of cardio. This gives him such bad PEM that his condition becomes "moderate." For the next 6 months, he can only do 1min of cardio without triggering PEM.

My suspicion is that some people develop *subclinical ME/CFS* immediately after their COVID infection. It's mild enough that they don't accidentally trigger PEM in their day-to-day life. And it's mild enough that it'll go away within weeks/months if they avoid PEM.

But if they exercise, and this *will* trigger PEM. After triggering PEM for the first time, their ME/CFS will progress from "subclinical" to "mild" (or possibly "moderate").

I expect most people don't have this "subclinical ME/CFS" phase post-COVID. But it's hard to tell *who* is going through it unless they trigger PEM. So that's where the danger lies.

If I were able-bodied, and I just caught COVID, I wouldn't wait a few weeks and then jump right back into my normal exercise routine. I'd operate with the assumption that I may have subclinical ME/CFS. As in: I'd start really small, wait 48-72h, and then increase my exertion. (And I'd monitor my HR to make sure it's not getting abnormally high during exercise.)

If you get *zero* PEM from 5min of exercise, you probably won't get *life-changingly bad* PEM from 10min of exercise, and so on. You can re-introduce exertion gently enough that you'll have a chance to notice mild PEM before you trigger severe PEM.